A while ago, I made a post about an SCP idea I had involving hostile thaumaturgic fish creatures from South America. I was going to write the article as a standard write-up of a in-universe monster species with a narrative of them attacking their containment by the Foundation but I decide to hold it back to write something different from the evil-monsters-who-don't-like-people formula and came up with a scenario centered around a conflict involving the same fish monsters but now against are more crazier monster species; also from South America. Now I'm trying to figure out the rest of the aspects of this scenario. I thought up most of the basics, but I'm still trying to figure out how the fish monsters should fit into this. I still want them to show some malignant, or at least unfriendly and self-serving, intentions. The two thoughts I have is one where they seem ok at first and the Foundation helps them out but them betrays them and another where there are more obvious hints at malevolence but the Foundation helps them anyway to get rid of the other monster species because they are more violent and don't respond to attempts a diplomacy like in a sort of Lawful Evil vs Chaotic Evil stance. Any advice?

The two thoughts I have is one where they seem ok at first and the Foundation helps them out

Why would the Foundation help an anomalous species/subspecies?

and another where there are more obvious hints at malevolence but the Foundation helps them anyway to get rid of the other monster species because they are more violent and don't respond to attempts a diplomacy like in a sort of Lawful Evil vs Chaotic Evil stance.

This sounds more like the Global Occult Coalition than the Foundation. But still, why are either of those two helping a race that should be contained (at least, in the Foundation's view)?

Also, how do you plan on fitting this in with a scip article? I see this working out as more of a tale, although incident reports could also be fitted into a scip article in order to make it work.

evil-monsters-who-don't-like-people formula and came up with a scenario centered around a conflict involving the same fish monsters but now against are more crazier monster species; also from South America.

This still fits with the monsters who don't like people formula, I feel. There is no bigger story in place here other than two species being against each other.

Why should the Foundation waste resources on trying to help these creatures rather than containing them (or even containing the scenario as a whole)?

This is a borderline case, but since the OP is asking for assistance regarding a particular part of their concept, and not whether the concept is good enough to be drafted, I think it's alright in Questions Desk.